1. Field of the Invention
The present invention relates to a system and method for controlling a state of charge for an energy storage system.
2. Background Art
With the ever increasing need to produce vehicles that are more fuel efficient, hybrid electric vehicles (HEV's) have provided an improvement in fuel economy over many conventional vehicles that utilize only an internal combustion engine to drive the vehicle. One of the primary advantages of an HEV is that it allows the vehicle to be powered by one or more electric motors under certain operating conditions. For example, if the speed of the vehicle is relatively moderate, and the battery or other electrical power sources sufficiently charged, the engine may be shut off, and the vehicle powered exclusively by the electric motors. As operating conditions change, the engine may be started to provide additional power, and if needed, operate a generator to charge the battery.
In order to ensure that the battery has enough power to perform desired functions—e.g., operate a generator to start the engine, or provide power to a motor to provide torque to the vehicle wheels—a state of charge of the battery is maintained within a predetermined range. For example, if the state of charge drops below a certain value, the engine may be started to drive a generator to charge the battery. Operation of the engine consumes fossil fuel, and produces undesirable emissions.
Conversely, if the state of charge rises above a certain level, additional charging may be undesirable. In such a case, additional charging of the battery is rejected. Therefore, if the vehicle is slowed or stopped—a situation that may typically call for the capture of regenerative braking energy—the vehicle's friction brakes may be used so that the battery is not overcharged. This is inefficient, as the energy available from the braking operation is lost.
Therefore, a need exists for a system and method for controlling the state of charge of an energy storage system, such as a battery, so that it is allowed to drop below some predetermined baseline if it is known or contemplated that charging operations will be occurring. Similarly, it would be desirable for the system and method to allow the state of charge to rise above the baseline if it is known or contemplated that the stored energy will be used.